Exhaust purifying device of internal combustion engine

ABSTRACT

An upstream catalyst and a downstream catalyst are arranged in series and are housed in a common casing disposed in an engine exhaust passage. The upstream catalyst including a NOx storage-reduction catalyst that adsorbs NOx contained in incoming exhaust gas when the air-fuel ratio of the incoming exhaust gas is lean, and releases and reduces the adsorbed NOx when the air-fuel ratio of the incoming exhaust gas becomes rich, and the downstream catalyst includes a three-way catalyst. The upstream catalyst has a higher oxidizing capability than the downstream catalyst, and the downstream catalyst has a higher reducing capability than the upstream catalyst. The upstream catalyst has a multi-layer structure including an upper layer and a lower layer, and is prepared such that the upper layer has a higher oxidizing capability than the lower layer, and the lower layer has a higher reducing capability than the upper layer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an exhaust purifying device of an internal combustion engine.

2. Description of the Related Art

In a known example of internal combustion engine (as disclosed in, for example, Japanese Patent Application Publication No. 11-44234 (JP-A-11-44234)), a NOx storage-reduction catalyst, which adsorbs NOx contained in exhaust gas flowing into the catalyst when the air-fuel ratio of the exhaust gas is lean, and releases and reduces the adsorbed NOx when the air-fuel ratio of the exhaust gas becomes rich, is disposed in an exhaust passage of the engine (i.e., lean-burn engine) in which an air-fuel mixture having a lean air-fuel ratio is normally burned. In this type of engine, the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is temporarily switched to the rich side when the NOx stored in the NOx storage-reduction catalyst is to be released and reduced. In this internal combustion engine, NOx contained in the exhaust gas is adsorbed by and stored in the NOx storage-reduction catalyst. The amount of NOx stored in the NOx storage-reduction catalyst gradually increases with the passage of time. Thus, the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is temporarily switched to the rich side before the NOx storage-reduction catalyst is saturated with NOx, so that the NOx stored in the NOx storage-reduction catalyst is released and reduced. In this case, the air-fuel ratio in the internal combustion engine, for example, is controlled to be rich (i.e., smaller than the stoichiometric ratio) so that the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is switched to the rich side.

In another known example of internal combustion engine (as disclosed in, for example, Japanese Patent Application Publication No. 2006-291812 (JP-A-2006-291812)), an upstream catalyst and a downstream catalyst are arranged in series with each other and are housed in a common casing disposed in an engine exhaust passage, and each of the upstream catalyst and the downstream catalyst has a single-layer structure or a multi-layer structure.

Since fuel consumption increases with an increase in the frequency at which the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is switched to the rich side, it is preferable, in terms of reduction of the fuel consumption, that the NOx storage-reduction catalyst has the highest possible NOx adsorbing capability or storage capacity. However, there is a limit to the space in which the NOx storage-reduction catalyst is installed, and it is therefore necessary to increase or enhance the NOx adsorbing capability of the NOx storage-reduction catalyst while minimizing the dimensions or capacity of the NOx storage-reduction catalyst.

Immediately after the air-fuel ratio of the exhaust gas flowing into the NOx storage-reduction catalyst is switched to the rich side, a large amount of NOx may be discharged from the NOx storage-reduction catalyst without being reduced. In this case, emissions of NOx need to be reduced.

To solve the above-described problems, an additional catalyst may be disposed upstream or downstream of the NOx storage-reduction catalyst, or the NOx storage-reduction catalyst may have a multi-layer structure, i.e., may be constructed of two or more layers, as disclosed in JP-A-2006-291812. However, the state of the art does not provide satisfactory solutions to the above problems.

SUMMARY OF THE INVENTION

The invention provides an exhaust purifying device of an internal combustion engine in which a NOx storage-reduction catalyst exhibits a high NOx adsorbing capability and a high NOx conversion efficiency.

According to one aspect of the invention, there is provided an exhaust purifying device of an internal combustion engine wherein an upstream catalyst and a downstream catalyst are arranged in series with each other and are housed in a common casing disposed in an engine exhaust passage, and wherein the upstream catalyst comprises a NOx storage-reduction catalyst that adsorbs NOx contained in incoming exhaust gas when the air-fuel ratio of the incoming exhaust gas is lean, and releases and reduces the adsorbed NOx when the air-fuel ratio of the incoming exhaust gas becomes rich, and the downstream catalyst comprises one of a three-way catalyst and a NOx storage-reduction catalyst. In the exhaust purifying device, the upstream catalyst and the downstream catalyst are prepared such that the upstream catalyst has a higher oxidizing capability than the downstream catalyst, and such that the downstream catalyst has a higher reducing capability than the upstream catalyst, and the upstream catalyst has a multi-layer structure including an upper layer and a lower layer, and is prepared such that the upper layer has a higher oxidizing capability than the lower layer, and such that the lower layer has a higher reducing capability than the upper layer.

In the exhaust purifying device as described above, each of the upper layer and the lower layer of the upstream catalyst may contain a noble-metal catalyst comprising at least one selected from platinum (Pt), palladium (Pd), osmium (Os), gold (Au), rhodium (Rh), iridium (Ir), and ruthenium (Ru), and a NOx absorbent comprising at least one selected from alkali metals, alkaline earths, and rare earths.

In the exhaust purifying device as described above, the upper layer of the upstream catalyst may contain, as the noble-metal catalyst, at least one selected from platinum (Pt), palladium (Pd), osmium (Os), and gold (Au), and the lower layer of the upstream catalyst may contain, as the noble-metal catalyst, at least one selected from rhodium (Rh), iridium (Ir), and ruthenium (Ru).

Also, the downstream catalyst may have a multi-layer structure including an upper layer and a lower layer, and may be prepared such that the upper layer has a higher reducing capability than the lower layer, and the lower layer has a higher oxidizing capability than the upper layer.

Furthermore, rhodium (Rh) may be used as a noble-metal component of the upper layer of the downstream catalyst, and platinum (Pt) may be used as a noble-metal component of the lower layer of the downstream catalyst.

The downstream catalyst may have a single-layer structure.

Furthermore, the downstream catalyst may contain rhodium (Rh) and platinum (Pt) as noble-metal components.

In the exhaust purifying device as described above, the air-fuel ratio in the internal combustion engine may be normally set to a lean air-fuel ratio that is larger than a stoichiometric ratio, and, when NOx stored in the NOx storage-reduction catalyst is to be released and reduced, the air-fuel ratio of exhaust gas flowing into the NOx storage-reduction catalyst may be temporarily controlled to a rich air-fuel ratio that is smaller than the stoichiometric ratio.

Furthermore, the air-fuel ratio in the internal combustion engine, may be temporarily controlled to the stoichiometric ratio, depending on engine operating conditions.

With the above arrangements, the NOx adsorbing capability and NOx conversion efficiency of the NOx storage-reduction catalyst can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements, and wherein:

FIG. 1 is an overall view of an internal combustion engine;

FIG. 2 is a cross-sectional view of a NOx storage-reduction catalyst;

FIG. 3A and FIG. 3B are cross-sectional views of a surface portion of a catalyst support;

FIG. 4 is an enlarged cross-sectional view of the NOx storage-reduction catalyst;

FIG. 5A through FIG. 5C are views showing various examples of NOx storage-reduction catalysts;

FIG. 6A and FIG. 6B are views showing various examples of three-way catalysts;

FIG. 7 is a view useful for explaining a predetermined load factor KLX;

FIG. 8 is a flowchart illustrating an engine operation control routine;

FIG. 9A through FIG. 9C are view showing various experimental results; and

FIG. 10 is a view useful for explaining a peak value of a discharged NOx amount.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 illustrates the case where the invention is applied to a spark ignition type internal combustion engine. The invention may also be applied to a compression ignition type internal combustion engine.

Referring to FIG. 1, the spark ignition type internal combustion engine includes an engine body 1, cylinder block 2, cylinder head 3, piston 4, combustion chamber 5, intake valve 6, intake port 7, exhaust valve 8, exhaust port 9, and a spark plug 10. The intake port 7 of each cylinder is connected to a surge tank 12 via a corresponding intake branch pipe 11. The surge tank 12 is connected to an air cleaner 14 via an intake duct 13. An air flow meter 15 and a throttle valve 17 adapted to be driven by a step motor 16 are disposed in the intake duct 13. A fuel injection valve 18 is mounted to the intake port 7 of each cylinder. The fuel injection valve 18 for each cylinder is connected to a common rail 19, and the common rail 19 is connected to a fuel tank 21 via a fuel pump 20 capable of controlling the amount of fuel delivered therefrom. A fuel pressure sensor 22 is mounted to the common rail 19, and the amount of fuel delivered from the fuel pump 20 is controlled so that the fuel pressure in the common rail 19 becomes equal to a target pressure.

On the other hand, the exhaust port 9 of each cylinder is connected to a casing 25 via an exhaust manifold 23 and an exhaust pipe 24, and the casing 25 is connected to an exhaust pipe 26. An air-fuel ratio sensor 27 is mounted in the exhaust pipe 24, and a catalyst 28 is housed in the casing 25.

An electronic control unit 30 consists of a digital computer, and includes ROM (read-only memory) 32, RAM (random access memory) 33, CPU (microprocessor) 33, input port 35, and output port 36, which are connected to each other via a bidirectional bus 31. A load sensor 40 that produces an output voltage proportional to the amount of depression of an accelerator pedal 39 is connected to the accelerator pedal 39. The input port 35 receives output voltages of the air flow meter 15, fuel pressure sensor 22, air-fuel ratio sensor 27 and the load sensor 40, via corresponding A/D converters 37. A crank angle sensor 41 produces an output pulse each time the crankshaft rotates, for example, 30°, and the output pulse is transmitted to the input port 35. The CPU 34 calculates the engine speed Ne, based on the output pulses received from the crank angle sensor 41. On the other hand, the output port 36 is connected to the spark plug 10, step motor 16, fuel injection valve 18, and the fuel pump 20, via corresponding driving circuits 38.

The catalyst 28 includes an upstream catalyst 28U and a downstream catalyst 28D which are arranged in series with each other in the casing 25. In one embodiment of the invention, the upstream catalyst 28U consists of a NOx storage-reduction catalyst, and the downstream catalyst 28D consists of a three-way catalyst. However, the downstream catalyst 28D may consist of a NOx storage-reduction catalyst. In this embodiment of the invention, the capacity of the upstream catalyst 28U is made equal to or larger than that of the downstream catalyst 28D. However, the capacity of the upstream catalyst 28U may be made smaller than that of the downstream catalyst 28D.

FIG. 2 illustrates the structure of the upstream catalyst, or NOx storage-reduction catalyst 28U. In the embodiment shown in FIG. 2, the NOx storage-reduction catalyst 28U has a honeycomb structure, and includes a plurality of exhaust gas channels 51 that are separated from each other by thin partition walls 50. A catalyst support 55 made of, for example, alumina is loaded on the opposite surfaces of each partition wall, or substrate 50. FIG. 3A and FIG. 3B schematically illustrate a cross-section of a surface portion of the catalyst support 55. As shown in FIG. 3A and FIG. 3B, a noble-metal catalyst 56 is supported, while being scattered, on the surface of the catalyst support 55, and a layer of a NOx absorbent 57 is formed on the surface of the catalyst support 55.

At least one selected from platinum (Pt), palladium (Pd), osmium (Os), gold (Au), rhodium (Rh), iridium (Ir), and ruthenium (Ru) is used as the noble-metal catalyst 56. As a component that constitutes the NOx absorbent 57, at least one selected from alkali metals, such as potassium (K), sodium (Na), and cesium (Cs), alkaline earths, such as barium (Ba) and calcium (Ca), and rare earths, such as lanthanum (La) and yttrium (Y), is used.

Where the ratio of air and fuel (hydrocarbon) supplied into the engine intake passage, combustion chamber 5 and the exhaust passage upstream of the NOx storage-reduction catalyst 28U is referred to as “air-fuel ratio of exhaust gas”, the NOx absorbent 57 performs NOx absorbing and releasing functions to absorb NOx when the air-fuel ratio of exhaust gas is lean, and release the absorbed NOx when the concentration of oxygen in the exhaust gas is reduced.

In the case where platinum (Pt) is used as the noble-metal catalyst 56, and barium (Ba) is used as a component that constitutes the NOx absorbent 57, by way of example, when the air-fuel ratio of exhaust gas is lean, namely, when the concentration of oxygen in the exhaust gas is high, NOx contained in the exhaust gas is oxidized into NO₂ on the platinum (Pt) 56 as shown in FIG. 3A, and NO₂ is then absorbed into the NOx absorbent 57, to be dispersed in the form of nitrate ions NO₃ in the NOx absorbent 57 while combining with barium carbonate (BaCO₃). In this manner, NOx is absorbed into the NOx absorbent 57. NO₂ is produced on the surface of the platinum (Pt) 56 as long as the concentration of oxygen in the exhaust gas is sufficiently high, and NO₂ is absorbed into the NOx absorbent 57 to form nitrate ions NO₃ as long as the NOx absorbing capability of the NOx absorbent 57 is not saturated.

If the air-fuel ratio of the exhaust gas turns rich, on the other hand, the concentration of oxygen in the exhaust gas is reduced, and the reaction proceeds in the reverse direction (NO₃→NO₂), so that nitrate ions NO₃ in the NOx absorbent 57 are released in the form of NO₂ from the NOx absorbent 57, as shown in FIG. 3B. Then, the released NOx is reduced by unburned HC and CO contained in the exhaust gas.

In this embodiment of the invention, the NOx storage-reduction catalyst 28U has a multi-layer structure including an upper layer 28UU and a lower layer 28UL, as shown in FIG. 4. Namely, the lower layer 28UL and the upper layer 28UU are successively laminated on the substrate 50. In this case, each of the upper layer 28UU and the lower layer 28UL provides a NOx storage-reduction catalyst, namely, includes the above-described noble-metal catalyst 56 and NOx absorbent 57. An additional layer may be provided between the upper layer 28UU and the lower layer 28UL, or between the lower layer 28UL and the catalyst support 55.

At least one selected from noble metals having a high oxidizing capability, such as platinum (Pt), palladium (Pd), osmium (Os), and gold (Au), is used as the noble-metal catalyst 56 of the upper layer 28UU. On the other hand, at least one selected from noble metals having a high reducing capability, such as rhodium (Rh), iridium (Ir), and ruthenium (Ru), is used as the noble-metal catalyst 56 of the lower layer 28UL. In this case, a noble metal having a high reducing capability is not contained in the upper layer 28UU.

FIG. 5A through FIG. 5C show various examples of the noble-metal catalysts 56 of the upper layer 28UU and the lower layer 28UL. As the noble-metal catalyst 56 of the upper layer 28UU, platinum (Pt) is used in the example of FIG. 5A, and palladium (Pd) is used in the example of FIG. 5B, while platinum (Pt) and palladium (Pd) are used in the example of FIG. 5C. On the other hand, rhodium (Rh) is used as the noble-metal catalyst 56 of the lower layer 28UL in all of the examples of FIG. 5A-FIG. 5C.

If the noble-metal catalysts 56 of the upper layer 28UU and lower layer 29UL are selected in the above manners, the oxidizing capability of the upper layer 28UU is made higher than that of the lower layer 28UL, and the reducing capability of the lower layer 28UL is made higher than that of the upper layer 28UU.

In the meantime, the downstream catalyst, or three-way catalyst 28D also has a honeycomb structure, like the NOx storage-reduction catalyst 28U, and includes a plurality of exhaust gas channels that are separated from each other by thin partition walls. A catalyst support made of, for example, alumina is loaded on the opposite surfaces of each partition wall, and a catalyst component including a noble-metal component is supported on the surface of the catalyst support.

In one embodiment of the invention, the three-way catalyst 28D has a multi-layer structure including an upper layer 28DU and a lower layer 28DL. In this case, each of the upper layer 28DU and the lower layer 28DL provides a three-way catalyst.

In the three-way catalyst 28D, at least one selected from noble metals having a high reducing capability is used as a noble-metal component of the upper layer 28DU, and at least one selected from noble metals having a high oxidizing capability is used as a noble-metal component of the lower layer 28DL. In the example shown in FIG. GA, rhodium (Rh) is used as the noble-metal component of the upper layer 28DU, and platinum (Pt) is used as the noble-metal component of the lower layer 28DL.

If the noble-metal components of the upper layer 28DU and the lower layer 28DL are selected as described above, the reducing capability of the upper layer 28DU is made higher than that of the lower layer 28DL, and the oxidizing capability of the lower layer 28DL is made higher than that of the upper layer 28DU.

Alternatively, the three-way catalyst 28D may have a single-layer structure. In this case, at least a noble metal having a high reducing capability is used as a noble-metal component of the three-way catalyst 28D. In addition, a metal having a high oxidizing capability may be used or may not be used. In the example shown in FIG. 6B, rhodium (Rh) and platinum (Pt) are used as noble-metal components of the three-way catalyst 28D.

If the noble-metal catalysts 56 of the upstream catalyst or NOx storage-reduction catalyst 28U and the noble-metal component(s) of the downstream catalyst or three-way catalyst 28D are selected as described above, the oxidizing capability of the NOx storage-reduction catalyst 28U is made higher than that of the three-way catalyst 28D, and the reducing capability of the three-way catalyst 28D is made higher than that of the NOx storage-reduction catalyst 28U.

In this embodiment of the invention, the upstream catalyst or NOx storage-reduction catalyst 28U and the downstream catalyst or three-way catalyst 28D are independently supported on the respective substrates, and these substrates are coupled in series with each other, thereby to form the catalyst 28. The NOx storage-reduction catalyst 28U may be supported on an upstream portion of a common substrate, and the three-way catalyst 28D may be supported on a downstream portion of the substrate.

The NOx storage-reduction catalyst 28U having a multi-layer structure is manufactured, for example, in the following manner. Here, the manufacturing method will be explained with regard to the case where rhodium (Rh) is used as the noble-metal catalyst 56 of the lower layer 28UL, and platinum (Pt) is used as the noble-metal catalyst 56 of the upper layer 28UU. Initially, a slurry is prepared in which support powder that forms the catalyst support of the lower layer 28UL and rhodium powder are dispersed, and the slurry is applied onto a substrate. In this case, zirconium (Zr), alumina (Al₂O₃), ceria (CeO₂), ZrO₂-Al₂O₃, ZrO₂-Al₂O₃-TiO₂, for example, may be used as the catalyst support of the lower layer 28UL. The rhodium powder is formed from PM powder, and is dispersed in the form of nitrate or acetate in the slurry. The viscosity of the slurry is preferably around 30%, for example, and the amount of coating preferably ranges from 50 g/L to 200 g/L. Then, drying (200° C., 2 hours) and firing (400° C., 4 hours) are conducted, so that the lower layer 28UL is formed.

Subsequently, a slurry is prepared in which support powder that forms the catalyst support of the upper layer 28UU and platinum powder are dispersed, and the slurry is applied onto the lower layer 28UL. In this case, zirconium (Zr), alumina (Al₂O₃), ceria (CeO₂), Al₂O₃-CeO₂, ZrO₂-Al₂O₃, or ZrO₂-Al₂O₃-TiO₂, for example, may be used as the catalyst support of the upper layer 28UU. The platinum powder is dispersed in the form of nitrate or acetate, such as tetrachroloplatinum or dinitroplatinum, in the slurry. The viscosity of the slurry is preferably around 30%, for example, and the amount of coating preferably ranges from 50 g/L to 200 g/L. Then, drying (200° C., 2 hours) and firing (400° C., 4 hours) are conducted, so that the upper layer 28UU is formed. In another method, a catalyst support may be first formed on the lower layer 28UL, and the catalyst support may be impregnated with an aqueous solution of tetrachroloplatinum or dinitroplatinum.

The three-way catalyst 28D having a multi-layer structure may also be manufactured in a manner similar to the NOx storage-reduction catalyst 28U.

In the embodiment of the invention, when the engine operates at a low load with the engine load factor KL being smaller than a predetermined or preset load factor KLX as shown in FIG. 7, a lean-mode operation is performed in which an air-fuel mixture having a lean air-fuel ratio is burned. When the engine operates at a high load with the engine load factor KL being larger than the predetermined load factor KLX, a stoichiometric-ratio operation is performed in which an air-fuel mixture having the stoichiometric ratio is burned. Here, the engine load factor KL represents the proportion of the engine load to the full load. In this case, it may also be said that an internal combustion engine that normally operates in a lean mode (i.e., a lean-burn engine) is temporarily switched to a stoichiometric-ratio operation, depending on the engine operating conditions.

Thus, when the engine operates in a lean mode, the air-fuel ratio of exhaust gas flowing into the NOx storage-reduction catalyst 28U becomes lean, and NOx contained in the exhaust gas is adsorbed by and stored in the NOx storage-reduction catalyst 28U. If the lean-mode operation continues to be performed, however, the NOx storage-reduction catalyst 28U adsorbs NOx to the full NOx adsorbing capability (namely, the NOx storage-reduction catalyst 28U is saturated with NOx adsorbed thereon), whereby the NOx storage-reduction catalyst 28U becomes unable to adsorb NOx any more. In the embodiment of the invention, therefore, the air-fuel ratio of the exhaust gas is temporarily made rich before the NOx storage-reduction catalyst 28U reaches the full NOx adsorbing capability (i.e., before the NOx storage reduction catalyst 28U is saturated with NOx), so that NOx is released from the NOx storage-reduction catalyst 28U, and reduced by HC, CO in the exhaust gas, into N₂, or the like.

Namely, in the embodiment of the invention, the amount of NOx adsorbed per unit time by the NOx storage-reduction catalyst 28U is stored in advance in the ROM 32, in the form of a map as a function of engine operating conditions, such as the engine load factor KL and the engine speed Ne. By integrating the NOx amount, a total value SN of the amount of NOx stored in the NOx storage-reduction catalyst 28U is calculated. Then, each time the total value SN of the stored NOx amount exceeds the upper limit MAX, a rich-mode operation is temporarily performed in which an air-fuel mixture having a rich air-fuel ratio is burned. As a result, NOx is released from the NOx storage-reduction catalyst 28U, and is reduced.

FIG. 8 illustrates a routine for implementing engine operation control according to the embodiment of the invention. This routine is executed as an interrupt at predetermined time intervals.

Referring to FIG. 8, it is initially determined in step 100 whether the engine load factor KL is larger than the predetermined load factor KLX (FIG. 7). If KL≦KLX, the control proceeds to step 101 in which a lean-mode operation is performed. In the following step 102, the total value SN of the stored NOx amount is calculated. In the following step 103, it is determined whether the total value SN of the stored NOx amount is larger than the upper limit MAX. If SN≦MAX, the current cycle of the routine of FIG. 8 ends, and the lean-mode operation is continued. If SN>MAX, on the other hand, the control proceeds to step 104, and a rich-mode operation is performed, for example, for a given period of time. In the following step 105, the total value SN of the stored NOx amount is cleared. If it is determined in step 100 that the engine load factor KL is larger than the predetermined load factor KLX, the control proceeds to step 106 in which a stoichiometric-ratio operation is performed.

According to the embodiment of the invention, the NOx adsorbing capability of the catalyst 28 or the NOx storage-reduction catalyst 28U can be enhanced.

FIG. 9A shows experimental results on the NOx storage capacity ST of the catalyst 28. In Comparative Example Ca shown in FIG. 9A, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a single-layer structure, and platinum (Pt) and rhodium (Rh) are used as a noble-metal catalyst. In Example Ea1, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a double-layer structure, and platinum (Pt) is used as a noble-metal catalyst of the upper layer while rhodium (Rh) is used as a noble-metal catalyst of the lower layer. In Example Ea2, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a double-layer structure, and platinum (Pt) and palladium (Pd) are used as a noble-metal catalyst of the upper layer while rhodium (Rh) is used as a noble-metal catalyst of the lower layer.

As is understood from FIG. 9A, the NOx storage capacity ST of the catalyst 28 is relatively large in Examples Ea1, Ea2, and is larger in Example Ea2 than in Example Ea1. This may be because the NOx storage-reduction catalyst has a multi-layer structure, namely, consists of two layers. Accordingly, the frequency at which the air-fuel ratio of exhaust gas flowing into the catalyst 28 is switched to the rich side (on which the air-fuel ratio is smaller than the stoichiometric ratio) can be reduced, and fuel consumption (i.e., the amount of fuel consumed) can be reduced.

When the air-fuel ratio A/F of the exhaust gas flowing into the catalyst 28 is switched to the rich side as shown in FIG. 10, the amount EXN of NOx discharged from the catalyst 28 per unit time rapidly increases, reaches its peak value PKN, and then decreases. In the embodiment of the invention, the peak value PKN of the discharged NOx amount EXN can be reduced.

FIG. 9B shows experimental results on the peak value PKN of the discharged NOx amount of the catalyst 28. In Comparative Example Cb1 shown in FIG. 9B, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a single-layer structure, and platinum (Pt) and rhodium (Rh) are used as a noble-metal catalyst. In Comparative Example Cb2, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a double-layer structure, and platinum (Pt) is used as a noble-metal component of the upper layer while rhodium (Rh) is used as a noble-metal component of the lower layer. In Example Eb, the catalyst 28 consists of an upstream catalyst and a downstream catalyst. The upstream catalyst consists of a NOx storage-reduction catalyst having a double-layer structure, and platinum (Pt) is used as a noble-metal catalyst of the upper layer while rhodium (Rh) is used as a noble-metal catalyst of the lower layer. The downstream catalyst consists of a three-way catalyst having a single-layer structure, and platinum (Pt) and rhodium (Rh) are used as noble-metal components.

As is understood from FIG. 9B, the peak value PKN of the discharged NOx amount is larger in Comparative Example Cb2 than that of Comparative Example Cb1. In Example Eb, however, the peak value PKN of the discharged NOx amount can be significantly reduced. This may be because NOx released from the upstream catalyst or NOx storage-reduction catalyst is reduced by the downstream catalyst. Accordingly, the NOx conversion efficiency can be held at a high level during lean-mode operation while assuring a large NOx storage capacity.

Furthermore, according to the embodiment of the invention, the NOx conversion efficiency EFFS of the catalyst 28 can be held at a high level when the air-fuel ratio of exhaust gas flowing into the catalyst 28 is substantially equal to the stoichiometric ratio, for example, during high-load operation.

FIG. 9C shows experimental results on the NOx conversion efficiency EFFS of the catalyst 28 when the air-fuel ratio of the incoming exhaust gas is substantially equal to the stoichiometric ratio. In Comparative Example Cc1 shown in FIG. 9C, the catalyst 28 consists solely of a NOx storage-reduction catalyst having a single-layer structure, and platinum (Pt) and rhodium (Rh) are used as a noble-metal catalyst. In Comparative Example Cc2, the catalyst 28 consists solely of a three-way catalyst having a double-layer structure, and rhodium (Rh) is used as a noble-metal catalyst of the upper layer while platinum (Pt) is used as a noble-metal catalyst of the lower layer. In Example Ec, the catalyst 28 consists of an upstream catalyst and a downstream catalyst. The upstream catalyst consists of a NOx storage-reduction catalyst having a double-layer structure, and platinum (Pt) is used as a noble-metal catalyst of the upper layer while rhodium (Rh) is used as a noble-metal catalyst of the lower layer. The downstream catalyst consists of a three-way catalyst having a single-layer structure, and platinum (Pt) and rhodium (Rh) are used as noble-metal components. Where INN represents the amount of NOx flowing into the catalyst 28 per unit time, and EXN represents the amount of NOx flowing out of the catalyst 28, the NOx conversion efficiency EFFS of the catalyst 28 may be expressed by the following equation:

EFFS=(INN−EXN)/INN

As is understood from FIG. 9C, the NOx conversion efficiency EFFS of Example Ec is higher than that of Comparative Example Cc1, and is substantially equal to that of Comparative Example of Cc2.

In the embodiment of the invention as described above, a rich-mode operation (i.e., operating the engine at a rich air-fuel ratio) is performed so as to make the air-fuel ratio of exhaust gas flowing into the NOx storage-reduction catalyst 28U rich. However, in an internal combustion engine provided with fuel injection valves through which fuel is directly injected into combustion chambers, the air-fuel ratio of the incoming exhaust gas may be made rich by injecting fuel into the combustion chamber during the expansion stroke or exhaust stroke. It is also possible to make the air-fuel ratio of the incoming exhaust gas rich by supplying a reductant or secondary fuel into an exhaust passage upstream of the NOx storage-reduction catalyst 28U.

In the embodiment of the invention as described above, a lean-mode operation is performed when the engine operates at a low load, and a stoichiometric-ratio operation is performed when the engine operates at a high load. However, a stoichiometric-ratio operation may also be performed during acceleration.

While the invention has been described with reference to example embodiments thereof, it is to be understood that the invention is not limited to the described embodiments or constructions. To the contrary, the invention is intended to cover various modifications and equivalent arrangements. In addition, while the various elements of the example embodiments are shown in various combinations and configurations, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention. 

1-5. (canceled)
 6. An exhaust purifying device for an internal combustion engine, comprising: an upstream catalyst and a downstream catalyst arranged in series with each other and housed in a common casing disposed in an engine exhaust passage, wherein: the upstream catalyst comprises a NOx storage-reduction catalyst that absorbs NOx contained in incoming exhaust gas when the air-fuel ratio of the incoming exhaust gas is lean, and releases and reduces the absorbed NOx when the air-fuel ratio of the incoming exhaust gas becomes rich, and the downstream catalyst comprises one of a three-way catalyst and a NOx storage-reduction catalyst; the upstream catalyst has a higher oxidizing capability than the downstream catalyst, and the downstream catalyst has a higher reducing capability than the upstream catalyst; the upstream catalyst achieves a higher oxidizing capability than the downstream catalyst and achieves a lower reducing capability than the downstream catalyst by a multi-layer structure including an upper layer and a lower layer, each of the upper layer and the lower layer of the upstream catalyst contains both of a noble-metal catalyst and a NOx absorbent comprising at least one selected from alkali metals, alkaline earths, and rare earths, the upper layer of the upstream catalyst contains, as the noble-metal catalyst, at least one selected from platinum (Pt), palladium (Pd), osmium (Os), and gold (Au); the lower layer of the upstream catalyst contains, as the noble-metal catalyst, at least one selected from rhodium (Rh), iridium (Ir), and ruthenium (Ru), the downstream catalyst has a multi-layer structure including an upper layer and a lower layer, and the upper layer has a higher reducing capability than the lower layer, and the lower layer has a higher oxidizing capability than the upper layer: rhodium (Rh) is used as a noble-metal component of the upper layer of the downstream catalyst; and platinum (Pt) is used as a noble-metal component of the lower layer of the downstream catalyst.
 7. The exhaust purifying device according to claim 6, wherein: the air-fuel ratio in the internal combustion engine is normally set to a lean air-fuel ratio that is larger than a stoichiometric ratio; and when NOx stored in the upstream NOx storage-reduction catalyst is to be released and reduced, the air-fuel ratio of exhaust gas flowing into the upstream NOx storage-reduction catalyst is temporarily controlled to a rich air-fuel ratio that is smaller than the stoichiometric ratio.
 8. The exhaust purifying device according to claim 7, wherein the air-fuel ratio in the internal combustion engine is temporarily controlled to the stoichiometric ratio, depending on engine operating conditions.
 9. The exhaust purifying device according to claim 6, wherein: the amount of NOx absorbed per unit time by the upstream NOx storage-reduction catalyst is stored in advance in a memory as a function of engine operating conditions; by integrating the NOx amount, a total value of the amount of NOx stored in the upstream NOx storage-reduction catalyst is calculated; and each time the total value of the stored NOx amount exceeds an upper limit, a rich-mode operation is temporarily performed in which an air-fuel mixture having a rich air-fuel ratio is burned.
 10. The exhaust purifying device according to claim 6, wherein the air-fuel ratio of the incoming exhaust gas is enriched by supplying a reductant or secondary fuel into an exhaust passage upstream of the NOx storage-reduction catalyst. 